This invention relates to seismic exploration and, more particularly, to a method which utilizes seismic data acquired by vertical seismic profile (VSP) exploration to tie seismic data shot using different types of seismic sources together.
To conduct seismic exploration of the earth, it is common practice to deploy geophones along a line of exploration of the surface of the earth for recording the vibrations of the earth in response to the generation of seismic energy. When the vibrations so recorded are caused by a seismic source activated at a known time and location, the recorded data can be processed by a computer in known ways to produce an image of the subsurface. The image thus produced is commonly interpreted by geophysicists to detect the possible presence of valuable hydrocarbons.
Seismograms are commonly recorded as digital samples representing the amplitude of a received signal as a function of time. Since seismograms typically correspond to a line of exploration along the surface of the earth, the acquired digital samples can be formed into x-t arrays with each sample in the array representing the amplitude of the seismic signal as a function of horizontal distance and time. When such arrays are visually reproduced, by plotting or the like, seismic sections are produced.
Numerous techniques for exploring the earth to acquire seismic data are well known. One such technique utilizes explosive sources for generating seismic energy into the earth. The explosive energy may be generated into the earth by an air gun, dynamite or the like, reflected off subsurface reflecting interfaces and detected by geophones located on the surface. A second well known technique utilizes a vibratory source for generating vertically-oriented seismic energy into the earth. Like explosive seismic exploration systems, vibrative energy propagates into the subsurface where it is reflected off subsurface reflecting interfaces and directed towards the surface for detection by geophones positioned on the surface. The vibrative exploration system may include, for example, the hydraulic vibrator marketed under the trademark "VIBROSEIS" by the Continental Oil Company.
The geophones positioned along the surface should record seismic data capable of being tied together when explosive and vibrative energy are alternately generated into the earth and reflected off the same interfaces. By "tied" data, it is intended to refer to seismic data which, despite corresponding to different types of seismic sources, includes the same amplitude and phase information for the subsurface formation. Such a result should occur because both types of seismic energy propagate through the same subsurface mediums and therefore should generate similar information regarding the characteristics of the subsurface. However, in many cases, the tieing of the two types of recorded data has not been possible due to the differences between the source waveforms of the generated explosive energy and vibrative energy.
Prior attempts to tie seismic data shot using different types of seismic sources attempted to correct for the differences between the source waveforms of the different types of seismic sources. These attempts proved to be unsatisfactory because, as the source waveform was unmeasurable, numerous assumptions regarding the seismic exploration were required. One such attempt to tie seismic data together relied on the assumption that the explosive and vibrative sources were minimum phase. Based on this assumption, spiking deconvolution would be applied to the acquired seismic data to determine what was incorrectly presumed to be the minimum phase amplitude spectrum of the source waveform. However, because seismic sources may not always be presumed to be minimum phase, particularly vibratory sources, the application of spiking deconvolution would often produce an incorrect minimum phase amplitude spectrum. Based on an often incorrect amplitude spectrum, the acquired data would then be corrected for differences in the source waveform in an attempt to tie seismic data from the different sources together.
A second prior attempt to tie seismic data from explosive and vibration sources together utilized the acquired vibrator data to estimate the amplitude spectrum of the vibrator source waveform. The minimum phase operator would then be determined from the estimation of the amplitude spectrum of the vibrator source waveform. Having determined the minimum phase operator, the acquired data could be corrected for differences in the source waveforms of the explosive and vibrative data to again attempt to tie the data from the two sources. However, estimates of the vibrator source waveform from the acquired vibrative seismic data were not always accurate. The vibrator source waveform error would then be propagated into the determination of the minimum phase operator and the correction of the acquired data, thereby resulting in a less than satisfactory "tie" of vibrative and explosive data.
Recently, the use of Vertical Seismic Profile (hereafter referred to as "VSP") exploration techniques have become increasingly popular. One method of conducting VSP exploration is to generate seismic energy, either vibrative or explosive, from a seismic source located on the surface into the earth. The generated energy is reflected by subsurface reflecting interfaces and detected by a geophone located in a borehole. Preferably, the source should be placed such that the subsurface reflection points generally lie in the plane containing the borehole and the source location. For a conventional VSP survey, the geophone would typically be moved to a new location for each shot with the distance between geophone locations being some constant distance such as 50 feet. See, for example, U.S. Pat. No. 4,627,036 issued to Wyatt and U.S. Pat. No. 4,597,464 issued to Chelminski for typical methods and apparatus for conducting VSP exploration of the subsurface.